1. Technical Field
The present invention relates to magnetic sensors and the like that measure minute magnetic fields generated by for example living bodies such as cardiac magnetic fields and neural magnetic fields by using an optical pumping method.
2. Related Art
The use of magnetic sensors that utilize optically pumped atomic magnetometers as a method of measuring minute magnetic fields has been considered. In such a method, a magnetic field is measured by optically pumping atoms of a gas and then detecting the magnetization of the atoms through the interactions between the atoms and the magnetic field. With such a method, there is no need to employ a large cooling mechanism as is necessary in the case of using superconducting quantum interference devices (SQUIDS) for example and the structure of the measurement device can be simplified and the cost can be reduced (For example refer to Reezaakou ni yoru genshibutsuri, Tsutomu Yabuzaki, Iwanami Shoten (2007), p. 29-57 and “Symmetry-recovering crises of chaos in polarization-related optical bistability”, M. Kitano, T. Yabuzaki, and T. Ogawa, Phys. Rev. A 29, 1288-1296 (1984)).
The basic structure of an optically pumped atomic magnetometer is illustrated in FIG. 7. In FIG. 7, a gas cell 103 is arranged between transparent heaters 101 and 102 composed of ITO or the like and laser light 105 is transmitted through these components. A gas (vapor) of alkali metal atoms such as cesium, rubidium or potassium atoms and a buffer gas such as helium, argon or nitrogen are enclosed within the gas cell 103 in suitable amounts. A magnetic field BO is a target of measurement and extends in a direction that is orthogonal to the direction in which the laser light 105 propagates. The amount of laser light 105 that is transmitted through the components is detected by a photodetector 104.
When BO=0, the alkali metal atoms absorb the circularly polarized laser light 105 and are thereby optical pumped, and the number N+ (population) of atoms possessing a magnetic moment that is parallel to the direction of propagation of the laser light 105 becomes larger than the number of atoms N− possessing a magnetic moment that is antiparallel to the direction of propagation of the laser light 105 and the gas enters a so-called spin polarized state. When N+ reaches a saturation level; due to repeated optical pumping, the atoms no longer readily absorb the laser light 105 (pumping light) and the proportion of light transmitted through the components becomes large. On the other hand, when BO has a finite value, since the magnetic moments of the atoms undergo Larmor precession about an axis parallel to the direction of BO, the difference between N+ and N− become substantially small. As a result, the atoms come to readily absorb the laser light 105 and the proportion of light transmitted through the components is reduced.
However, the magnetic fields generated by living bodies such as cardiac magnetic fields and neural magnetic fields are very weak, for example on the order of 100 pT, and so separating and detecting only the signal corresponding to the magnetic field of the living body from among for example noise generated by the output laser light 105 and electrical noise generated by the photodetector 104 and stages subsequent thereto is very difficult.